It is known to construct electrical heaters with electrical insulating material laminated on each side of a heating element. The heating element may be a resistance foil, a resistance wire, a film coated with electroconductive particles such as carbon black, or a woven glass fibre cloth impregnated with carbon black. Each of these constructions has its disadvantages.
Resistance foils and wires tend to produce hot spots and are thus prone to burn out because the usual zig-zag path produces a non-uniform distribution of the element in the layer containing the heating element. Foils, films and woven fabrics do not maintain a good bond to the insulating material. The stresses generated when the heater is used can cause delamination, voids in the material and consequent hot spots and eventually burn out of the heating element. In constructing the known heaters, air bubbles can be included in the lamination adjacent the heating element, leading to internal oxidation.
The present invention aims at the provision of an improved heating element.